BIOSYNTHESIS AND GENETIC-ENGINEERING OF LIGNIN

Lignin, a complex heteropolymer of cinnamyl alcohols, is, second to ce llulose, the most abundant biopolymer on Earth. Lignification has play ed a determining role in the adaptation of plants to terrestrial life. As all extracellular polymers, lignin confers rheological properties to plant tissues and participates probably in many other functions in cell and tissue physiology or in cell-to-cell communication. Economica lly, lignin is very important because it determines wood quality and i t affects the pulp and paper-making processes as well as the digestibi lity of forage crops. For all these reasons the lignin biosynthesis pa thway has been the subject of many studies. At present, most genes enc oding the enzymes involved in the biosynthesis of lignin have been clo ned and characterized. Various recent studies report on the alteration of the expression of these genes by genetic engineering, yielding pla nts with modified lignin. In addition, several mutants have been analy zed with changes in lignin content or lignin composition resulting in altered properties. Thanks to these studies, progress in the knowledge of the lignin biosynthesis pathway has been obtained. It is now clear that the pathway is more complex than initially thought and there is evidence for alternative pathways. A fine manipulation of the lignin c ontent and/or composition in plants is now achievable and could have i mportant economical and environmental benefits.